From the README:
The menu can be controlled by clicking the frame borders. Touching the inner
part of the screen is like a button press. In the game mode a click
in the upper right corner activates pause mode and the lower right
corner escapes to the menu.

The four corners of the screen and then the centre of the screen are 5 different "buttons". Trial and error means someone will eventually work out which is which and document it here.

From the README:
The menu can be controlled by clicking the frame borders. Touching the inner
part of the screen is like a button press. In the game mode a click
in the upper right corner activates pause mode and the lower right
corner escapes to the menu.

The four corners of the screen and then the centre of the screen are 5 different "buttons". Trial and error means someone will eventually work out which is which and document it here.